1. Field of the Invention
The present invention relates to a communication apparatus and a communication method for effecting communication of information of plural kinds among plural locations.
2. Related Background Art
In the field of digital communication line consisting of an information channel and a signal channel, the ISDN (integrated services digital network) has recently been standardized by the ITU-TS (former CCITT) and its commercial use is being promoted.
As a communication apparatus for handling the audio and video media through such ISDN services, there is known a multimedia communication apparatus exemplified by a television telephone. Such multimedia communication apparatuses execute communication, utilizing a common function among those of the respective apparatuses. It is therefore necessary, among such multimedia communication apparatuses, to have a common structure of the transmission frame for multiplexing the media signals such as of voice, image and other data on the communication channel. Such transmission frame structure is defined in the recommendation H.221 of ITU-TS. Also the various transmission modes between the mutually communicating terminals are defined by the recommendation H.242 of the ITU-TS.
FIG. 18 shows an example of the transmission frame structure defined by the recommendation H.221 of ITU-TS, wherein a frame synchronization signal FAS is composed of frame information, control information, alarm information, etc. A bit rate assignment signal BAS is composed, for example, of a terminal ability for variously structuring the channels in the frame. In FIG. 18, a structure of 80 bits in the vertical direction, corresponding to the octet numbers, is called a subchannel. The subchannels are numbers in succession from a subchannel #1 to a subchannel #8. The subchannel #8, in which the first 16 bits are assigned to the FAS and BAS mentioned above, is composed of the remaining 64 bits. The transmitted data of each medium are assigned in the unit of each subchannel. In case of data transmission in the frame structure according to the ITU-TS recommendation H.221, the FAS and BAS are added in each frame.
An even-numbered frame and an odd-numbered frame constitute a sub-multi frame, and 8 sub-multi frames constitute a multi frame. The FAS and BAS mentioned above are defined in the pair of the even- and odd-numbered frames, and a bit pattern of a frame synchronizing word, called FAW, is inserted in the FAS of the even frame and that of the odd frame. As shown in FIGS. 19A and 19B, the FAW bit pattern has "0011011" in the second to eighth octet numbers in the even frame and "1" in the second octet number in the odd frame.
In the following there will be considered a case of communication with the data of the above-explained frame structure, among the terminals connected in a ring shape as shown in FIG. 11.
A terminal A receives and reproduces the multimedia data defined by the above-explained frame structure and generated in terminals B and C. It also deletes the data of the terminal C, stores the multimedia data generated in the terminal A into an area formed in the frame by such deletion and transmits the data to a terminal E.
A terminal A receives and reproduces the multimedia data defined by the above-explained frame structure and generated in terminal B and C. It also deletes the data of the terminal C, stores the multimedia data generated in the terminal A into an area formed in the frame by such deletion and transmits the data to a terminal E.
Also, the terminal A receives and reproduces the multimedia data defined by the above-explained frame structure and generated in terminal D and E. It deletes the data of the terminal D, stores the multimedia data generated in the terminal A into an area formed in the frame by such deletion and transmits the data to the terminal B.
For generating the data to be transmitted, there is conceived a method of decoding encoded reception multimedia data, and effecting the encoding again to obtain the layout of the frame structure of the recommendation H.221, but such decoding and encoding are time-consuming. Also, such decoding and encoding apply considerable load on the CPU if they are executed in each terminal. Also, the data deterioration may result in such decoding and encoding.
For this reason, there has been conceived a configuration including a process system not effecting the decoding and reproduction, wherein, among the received multimedia data, those merely transmitted to the next terminal are processed by such process system with a change in the area within the frame (such process being hereinafter called turn-back transmission).
However, in the data transmission with the above-explained frame structure of the conventional multimedia communication apparatus, the data assigned to the subchannel #8 have a data rate different from that of other subchannels (smaller data rate because of the FAS/BAS area), so that, in case a partner terminal turns back the received data, the data received in the subchannel #8 cannot be assigned in other subchannels in the turn-back transmission.
Also, even if such assignment is made possible in the transmission, such transmitted data cannot be reproduced in the reception. For this reason, the communication by such turn-back transmission has not been possible.